I'm involved in my Church and as far as I know there's no actual cut off for Catholic nuns. The Church doesn't want it to become a retirement program, but as long as you're able to be useful they'll evaluate your application.

Nuns often work in Catholic schools as teachers or administration, or in the parish community programs. They want people who are competent and active, but there's no hard age limit. There's an active outreach for younger nuns, but age is not an automatic disqualification.